public Node decetCycle() {//判断是否有环，有环的话找到进入环的第一个节点
        Node fast = this.head;
        Node slow = this.head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (slow == fast) {
                break;
            }
            if (fast==null||fast.next==null){
                return null;
            }
        }
        fast = this.head;
        while (fast!=slow){
            fast=fast.next;
            slow=slow.next;
        }
   return fast;
    }